How long are the reading off before returning to normal? Or are they off as long as the other device is on?
What other symptoms are there?
Do we know if the gauges normally read correctly, e.g. have they been compared to resistance on sender?
Did the boat come to you this way? Did the problem suddenly develop? Did it develop slowly over time? Didnit appear at the same time another change happened or was made?
The odd thing is, unless I'm misremembering (which is very possible) is I think the resistance of most temp senders goes down as temp goes up and are part of the ground path for the gauge. So, I'm a little surprised the temp moves up. I forget if the gauges are a bimetallic ammeter, or similar, or if they are volt meters across a resister forming a voltage divider. But, either way, I'd expect a lower voltage to mean a lower temp.
The only reason I mention the above is that, if the guages stay off vs bouncing, and it is an old problem, tou msy want to check that the gauge is getting a real +12 and not in series with something else.
But, as others have said, 99% of the time it is corrosion, a bad connector, etc.